During recent years attempts to reduce the use of halogen-containing compounds have gained an increased interest, particularly in the field of pulp and paper making. Organic halogen in organic compounds are responsible for an increased halogen load in waste water as well as in paper and paper board. Epihalohydrin-based resins are halogen-containing organic compounds widely used as additives in the production of paper, for instance as wet-strength agents. Many methods have been developed for reducing the organic halogen content of epihalohydrin-based resins. European patent application 0512423 and U.S. Pat. Nos. 4,857,586 and 4,975,499 relate to the treatment of aqueous solutions of epihalohydrin-based resins with strong bases. European patent application 0510987 discloses enzymatic dehalogenation of halogen-containing compounds present in aqueous solutions of epihalohydrin-based resins. However, a major drawback with these methods is that they only reduce the organic halogen content but increase the inorganic halogen content in the form of halogen ions, whereby the total halogen content in the aqueous solution will remain constant. This is a serious limitation since organic halogen will be formed by reactions of the halogen ions with organic compounds present in the aqueous solution, in particularly if the pH of the product is lowered to below 7, especially 3-5, for improvement of storage stability.
WO 92/22601 reveals the possibility to remove both organic and inorganic halogen from epihalohydrin-based resins by passing an aqueous solution thereof through a strongly basic ion-exchange resin. A drawback with this process is the non-continuous operation which is due to the need to regenerate the ion exchange resin from time to time. Rinsing and regeneration or backwash of the resin also produces effluents, which still contain organic compounds causing problems in the waste water due to their chemical oxygen demand, and rather high salt load since chemicals for regeneration have to be used in excess.
The use of electrodialysis has been described in the literature on numerous occasions, see e.g. R. W. Baker et al, Membrane Separation Systems, Noyes Data Corp., 1991. Electrodialysis is a well established technique for desalination of brackish water for the production of potable water and table salt and it is most frequently used in processes involving inorganic material. However, according to U.S. Pat. Nos. 4,802,965 and 5,145,569, electrodialysis can also be used for removing salts from aqueous solutions of organic compounds.
Accordingly, it is an object of the present invention to provide a process for treating an aqueous solution of a nitrogen-containing epihalohydrin-based resin in order to produce an aqueous solution of a nitrogen-containing epihalohydrin-based resin having a reduced content of organic and inorganic halogen. It is further an object of the invention to provide a process as described above which can be carried out continuously. Another object of the present invention is to provide a process as described above which produces an aqueous solution having a reduced content of halogenated products and halogenated by-products. Still another object of the invention is to provide a process as described above which reduces the content of organic and inorganic halogen in the aqueous solution of a halogen-containing organic compound to levels lower than those obtainable by applying known methods.
The objects of the invention are achieved by a process as further defined in the claims. More specifically, the invention relates to a process for reducing the content of organic and inorganic halogen in an aqueous solution of a nitrogen-containing epihalohydrin-based resin by subjecting the aqueous solution to an electrodialysis treatment.